The present invention is directed to a tool cylinder belonging to a rotary machine for processing continuously at high speeds a web-like or sheet-like workpiece, such as, for instance, sheets of boards for the manufacture of packaging boxes. The particular operation referred to consists in forming a box blank out of a pre-printed sheet and this blank is then subsequently folded and glued so as to become a definite box.
For quality boxes, one clearly thinks of boxes containing the product and also serving as a publicity support and brand image of the product contained in the box. This is the case for the flip-top type cigarette boxes for luxurious cigarettes which are semi-rigid boxes of which the top bends backward in order to render the cigarettes very accessible. In order to obtain a quality product, it is easily understandable that every intermediate operation requires the same care. The creasing operation for preparing the future folding lines, for example, needs to be thin and precisely located with regard to the imprinted motif or printed pattern of the box. The cutting operation not only has to be accurate, but also neat in order not to leave any blurred or rough edges along the sides, as well as in the slightest corners. It is, thus, a primary importance to manufacture a male tool cylinder for cutting or creasing and a joint smooth counter-cylinder, both of a good quality and for performing such operations.
For the manufacture of liquid containers, cutting tool cylinders are presently well known. These cylinders comprise a carrying cylinder with grooves which engage one or several bases adapted as a portion of a hollow cylinder, which bases are provided with segments which the outer surfaces have been machined so as to have cutting blades. The position of the base on the carrying cylinder is fixed by means of catches or projections which are situated within the base and are set into grooves. The position of the segments on the base is given by bolts either running through these segments in order to be screwed into the base or crossing simultaneously the segments and the base in order to be threaded into the threadings of the carrying or carrier cylinder. The advantage of this device is that the segments are interchangeable. In other words, it is easy to change the shape of the liquid container, for example to modify its capacity by substituting, adding or taking away standard segments from the bases.
However, the accuracy of the position of the segments with regard to one another and to the pre-printed sheets seems to be unsatisfactory for finer jobs. These imprecisions or imperfections are caused, on the one hand, by the accumulated positioning errors of the bases with regard to the carrying cylinders and of the segments with regard to the bases. These imperfections are caused, on the other hand, by the fact that the segments are obtained independently from one another. An important offset is frequently noticed between two knives forming a right angle at the joining point of two segments. The outer diameter of the carrying cylinder can then be too small with regard to the total diameter of the tool cylinder, which situation can impair the rigidity of the assembly. Moreover, the setting of the position of the knives on every segment, as well as the position of the segment with regard to one another, is actually very time consuming and a tedious operation. In fact, standard segments are used as much as possible for the composition of complex diamond-shaped or star-shaped knives. Finally, taking into account the important number of segments used for the realization of an entire tool cylinder, their best possible setting becomes also long and tedious.
When even more precision is required for a very particular application, monobloc cylinders are directly engraved, preferably by electro-erosion. As may be gathered, the main drawback of this procedure consists in the very difficult correction of an error and the restoration of a broken knife. To this effect, it will be necessary to overlay the appropriate area with welded metal and to sharpen it by hand. Obviously, if the damage occurs with the monobloc cylinder installed in the machine, this requires a long and tedious restoration, which will cause the production from this machine to be in an undue down time.
Moreover, bending stresses which occur within the tool cylinder and which are associated with the counter-cylinder may cause resonance effects with high speed rotation inducing an uneven contact line between the tool cylinder and its counter-cylinder. In other words, it is frequent that neat cuts will appear at the two lateral ends of the cylinder but only partial cuts will be in the center area. In such a case, it will be necessary to bring both tool cylinders closer to one another, which causes a quick deterioration of the knives in the area adjacent the lateral ends where the overpressure will occur.